template<typename F, typename T>
T divdiff(F f, T t1, T t2) {
  return (f(t1) - f(t2)) / (t1 - t2);
};

template<typename F, typename T, typename... Args>
T divdiff(F f, T tstart, Args... t, T tend) {

  return (divdiff(f, tstart, t...) - divdiff(f, t..., tend))/ (tstart - tend);

};

double r = divdiff([](double x) { return 2 * x; }, 1.0, 2.0, 3.0);

note: candidate function not viable: requires 3 arguments, but 4 were provided
T divdiff(F f, T tstart, Args... t, T tend) {``